SEX-LINKAGE IN FOWLS ols 
tial sex-linkage be admitted, such crossing over must be possible.° 
If, on the other hand, we use the MM, Mm scheme we meet 
with no such. difficulties. In this case the heterochromosome 
contained in the female-producing egg is smaller than its mate in 
the male-producing egg, since it lacks the factor M (fig. 3),’ 
and the sex-formula is MM, Mm, two F’s being always present 
in both sexes, and contained, presumably, in some other pair 
of chromosomes. This would allow complete sex-linkage, for 
genes in that part of the M-chromosome having no homologue 
in the m-chromosome, and incomplete sex-linkage, for genes in 
the part having such a homologue. One might, of course evade 
this conclusion by assuming the condition represented in fig. 4. 
In this case the chromosomes would not conjugate evenly, and 
the part marked L could carry such factors as are completely 
sex-linked. | 
Of the converse case, where the male is heterozygous for sex 
there are not so many examples known. In Drosophila Mor- 
gan (710, 711, ’11 a, ete.) has reported numerous sex-linked factors. 
Miss Stevens (’08) has found here exactly the cytological con- 
ditions demanded by the experimental evidence. In man color- 
blindness follows this same scheme of inheritance, as do appar- 
ently several diseases (Bateson, 09; Morgan, ’11). Here, too, 
Guyer (’10) has reported cytological evidence that it is the male 
which is heterozygous. Finally we have the case of partial sex- 
linkage referred to above. Miss Stevens (11) has reported hetero- 
chromosomes in the male guinea-pig, and as that animal has 
been experimentally bred quite extensively I was led to look 
for sex-linkage in it. Perhaps the dwarf form studied by Miss 
Sollas (’09) is such a case. This is a recessive form, and has 
not been reared to maturity, or had not been in 1909, so that 
the case is not thoroughly worked out, but it seems to be most 
easily explainable as a case of partial sex-linkage. If we rep- 
6It should be noted that, if my view is correct, th sex chromosomes under dis- 
cussion are homologues, not of the Y and Y of Diptera, Hemiptera, etc., but of 
the M-bearing chromosomes in those groups. 
7 On this scheme two F’s are to be assumed to be always present in both sexes, 
probably situated in another pair of chromosomes. 
